The present invention relates to the measurement of radiation, especially visible radiation. More particularly, the present invention relates to a system and method for measuring the intensity of sunlight.
It is of interest to many branches of science and engineering to know the value of the intensity of the sun's direct rays. Instruments well known for measuring the sun's radiation include the calibrated silicon cell, the pyranometer, and the pyrheliometer. The pyranometer and the silicon cell have rather large angles of acceptance, and so respond to both the diffuse radiation incident thereon, and to the direct rays of the sun times the cosine of the angle of incidence. To determine the intensity of the direct rays only, these instruments are then oriented in a direction normal to the direct rays, and a reading is obtained. Another reading of just the diffuse radiation, as by shadowing the instrument, is also obtained. The latter is then subtracted from the former.
The pyrheliometer has a much smaller acceptance angle and so excludes substantially all of the diffuse radiation. It can therefore give a direct reading when aimed directly at the sun.
In all these cases, however, a measurement must be made in which the instrument is oriented so that it is normal to the sun's direct rays. For mulitiple and/or continuous measurements, therefore, constant tracking of the sun is required.
A need thus remains for a method and apparatus which can accurately measure the value of the intensity of the direct rays of the sun without requiring tracking. Such a device and method would preferably employ a minimum of parts, have no moving parts, and require no power for tracking. Advantageously, this would provide for a rugged, inexpensive, reliable device suitable for extended, unattended use, as in remote sites where tracking power would be unavailable.